Technical Field
The present invention relates to a method of packing silicon and a packing body, the primary packing object of which is polycrystalline silicon used as molten material when manufacturing single crystal silicon.
Related Art
As a method of manufacturing single crystal silicon, there is known the Czochralski method (hereunder, referred to as the CZ method). This CZ method has an advantage in that a large diameter high-purity silicon single crystal in a dislocation-free state or in a state of having very low lattice defects can be easily obtained.
In the CZ method: high-purity polycrystalline silicon is placed in a quartz crucible and melted in a furnace; the silicon melt is contacted with a wire-suspended seed crystal (silicon single crystal); and a silicon single crystal is pulled out gradually while rotating, to grow the silicon single crystal. At this time, in order to increase the volumetric efficiency of the quartz crucible to thereby improve silicon single crystal productivity, lump of polycrystalline silicon which has been cut and crushed from a polycrystalline silicon rod is loaded at high density.
However, since this lump of polycrystalline silicon is a brittle material, the edges of the cut surfaces and the edges of the crushed surfaces are often sharp. Consequently, when this is packed in a packing bag such as a polyethylene resin bag and transported, a cushioning material such as polystyrene foam, bubble cap, or plastic cardboard is used. However, vibrations still cause rubbing between the lump of polycrystalline silicon and the surface of the packing bag, and the lump of polycrystalline silicon and the packing body become pulverized in some cases. If fine powder of such polycrystalline silicon lump and polyethylene resin is brought into the above mentioned quartz crucible together with the lump of polycrystalline silicon, it causes crystal defects in the single crystal silicon produced after being pulled out of the quartz crucible, and consequently causes a reduction in the quality of a silicon single crystal.
Heretofore, in order to avoid such fine powder being generated from the packing bag, for example Patent Document 1 (Japanese Unexamined Patent Application, First Publication No. 2002-68725) proposes a transport method in which lump of polycrystalline silicon is brought into close contact with the packing bag and vacuum-packaged so that the polycrystalline silicon lump and the packing bag do not rub against each other. Moreover Patent Document 2 (Japanese Unexamined Patent Application, First Publication No. 2006-143552) proposes a method of reducing fine powder generated from the packing bag, by managing the area where the lump of polycrystalline silicon and the packing bag can come into contact with each other when packing lump of polycrystalline silicon.
Incidentally, the prime example of a cause of the fine powder generated from the polycrystalline silicon lump and the packing bag is rubbing between the polycrystalline silicon lump and the packing bag caused by vibrations in transport as mentioned above. Therefore, in the case where the methods disclosed in Patent Document 1 and Patent Document 2 are used, since the vibrations received by the polycrystalline silicon lump are not reduced, suppression of fine powder generation is limited. Moreover vibrations can be reduced to a certain degree by inserting a shock absorbing material such as polystyrene foam into the transporting case. However, there is a problem in that in order to reduce vibrations further, use of a large amount of polystyrene foam is necessary, and there is a cost for after-use processing and a negative impact on the environment.